Pulseless arterial flow and its physiologic consequences have become increasingly important with the development of techniques for prolonged total body circulatory assistance and single organ perfusion for preservation. In an acute anesthetized canine preparation, the effects of pulseless flow will be evaluated on organ blood flow distribution, intra-renal cortical flow distribution, renal cortical renin distribution, and reactivity in the mesenteric microcirculation. In chronic unanesthetized pulseless calves, blood flow distribution and organ function will be evaluated over a 14 day period. These data should provide important new information on the tolerance of prolonged pulseless perfusion, and the applicability of a centrifugal blood pump for prolonged circulatory assistance.